Abstract
A comprehensive understanding of chemical interactions underlying the network structure of chalcogenide materials is a crucial prerequisite for comprehending their microscopic structures, physicochemical properties, and capabilities for current or potential applications. However, for many chalcogenide materials, an inherent difficulty is often present in investigating their chemical properties, due to the involvement of delocalized bonding and non‐bonding (“lone‐pair”) electrons, which requires interaction mechanisms beyond that of conventional two‐center, two‐electron covalent bonding. Herein, some recent progress in the development of new interatomic interaction models for chalcogenides is reviewed, in particular focusing on the multi‐center hyperbonding model, proposed in an effort to resolve this issue. The capability of this model in elucidating a diversity of interesting material properties of phase‐change materials (PCMs) is highlighted, including Ge2Sb2Te5 (GST). These include the propensity of high coordination numbers of constituent atoms, linear triatomic bonding geometries with short and long bonds (often ascribed to the effect of a Peierls distortion), abnormally large Born effective charges of crystalline GST, large optical contrast between amorphous and crystalline GST, ultrafast crystallization speed of amorphous GST, and the chemical origin differentiating non‐PCM from PCM chalcogenide materials. Other bonding models for these materials are also briefly discussed.
Highlights
This is because, in most cases, the prerequisite material conditions for the non-volatile memory application are met only by this mate-Amorphous chalcogenide materials, containing heavy tellurium rial type
We focus here on the Ge2Sb2Te5 (GST) material, the prototypical phase-change materials (PCMs), the conclusions are quite general for other PCMs and non-PCM chalcogenides
With the help of this model, the chemical interactions constituting a-GST can be comprehensively described in terms of 2c/2e ordinary covalent bonds and 3c/4e hyperbonds
Summary
This is because, in most cases, the prerequisite material conditions for the non-volatile memory application are met only by this mate-. [+]Present address: Department of Engineering, University of Cambridge, Cambridge CB3 0FA, UK [++]Present address: Trinity College, University of Cambridge, requirement of a large optical contrast corresponds to there being a large difference in dielectric constants or Born effective charges (BECs) between the two phases, whereas the latter fast crystallization is associated with facile local structural changes or valencecharge redistribution in amorphous PCMs.[9] From a structural point of view, these two material requirements appear to be in conflict with each other, in that large (small) overall structural changes are anticipated for the former (latter) condition, respectively. Kohn–Sham (KS) orbitals were represented in terms of localized Wannier functions by unitary transformations, each of which is considered here as representing either a bond or LP, depending on the criteria for bond formation defined in the analysis.[9,17]
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